1. Field of the Invention
The present invention relates to wireless terminals for conducting wireless communications in a wireless LAN (Local Area Network), access points for communicating wirelessly to these wireless terminals, as well as wireless communication systems and methods provided therewith.
2. Description of the Related Art
In recent years, wireless LANs have become widespread as one type of computer communications network, and are now used with great popularity in offices, homes, and public places (for example, train stations, airports, and fast food restaurants). As is commonly known to those skilled in the art, wireless communication in such a wireless LAN is conducted between an access point (hereinafter, AP) and wireless terminals.
In this wireless LAN, a plurality of wireless terminals and APs communicate in a single shared space. Two or more devices among the plurality of wireless terminals and APs cannot send wireless signals simultaneously. Therefore, it is necessary to adjust transmitting timing for the wireless signals.
A method for adjusting the transmitting timing for wireless signals is stipulated in detail in the IEEE (Institute of Electrical and Electronics Engineers) 802.11 standard [“Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications” ANSI/IEEE Std 802.11, 1999 Edition.] in the non-patent document below.
In the IEEE 802.11 standard, a method referred to as the DCF (Distributed Coordination Function) is stipulated as a method for adjusting the transmitting timing for wireless signals.
This DCF method will now be simply described with reference to FIG. 20. The following description takes as an example the case wherein wireless communication between one AP and two wireless terminals is conducted using a single space. In addition, for the sake of brevity in the following description, transmission signals, including, for example, data signals and encoded voice signals, will be collectively referred to as data.
First, an AP transmits a Data1 signal addressed to a wireless terminal A (A101). Upon receiving a Data1 signal normally, the wireless terminal A waits a minimum period known as the SIFS (Short Inter Frame Space), and returns an ACK signal addressed to the AP (TA101). When the communication of the above ACK signal is completed, each device, in order to obtain next transmission rights, waits respective DIFS (Distributed Inter Frame Space, DIFS>SIFS)+random number (positive or zero) intervals (A102, TA102, TB101). In addition, during these respective intervals, each device respectively confirms that a wireless signal from other devices is not present.
For the purposes of this example, it is supposed that the random number of the wireless terminal A is the smallest, and therefore the wireless terminal A completes its waiting time first and obtains transmission rights. In so doing, the wireless terminal A transmits a Data2 signal addressed to the AP (TA103). Since the above-described random numbers of the AP and the wireless terminal B, respectively, were larger than that of the wireless terminal A, the wireless signal from the wireless terminal A is received by the AP and the wireless terminal B before their respective waiting time end. At this point, the AP and the wireless terminal B immediately halt waiting time receive said wireless signal, and each device subsequently determines, respectively, whether or not the wireless signal is addressed to itself.
The AP, upon receiving a Data2 signal normally, confirms that the signal is addressed to itself, and after subsequently waiting an SIFS interval as per the above-described, returns an ACK signal addressed to the wireless terminal A (A103), and notifies that a Data2 signal was received normally (S105). In a manner equivalent to the above-described case, when the transmission of this ACK signal is completed, the AP and the wireless terminals A and B again respectively wait (DIFS+random number) intervals (A104, TA104, TB102), and detect whether or not a receiving wireless signal is present during the waiting time. The device whose waiting time ends first, obtains transmission rights for the next transmission and transmits data.
Supposing hypothetically that the random numbers for the wireless terminal A accords with that for the wireless terminal B by chance, the wireless terminals A and B will respectively transmit a Data3 signal and a Data4 signal addressed to the AP simultaneously (TA105, TB103). As a result, these wireless signals will interfere with each other, and the AP will be unable to receive either wireless signal.
In order to avoid such interference, the IEEE 802.11 standard stipulates that a parameter known as the duration time, which reserves the transmission time for the data transmitted by wireless from the current time, is to be included in the IP packet header of the transmitted data.
A specific method for avoiding the above-described interference by utilizing this duration time is described in patent document 1 (Japanese Unexamined Patent Application Publication No. 2006-166114).
The method described in the above patent publication will now be simply described with reference to FIG. 21. For the sake of simplicity the following description also takes as an example the case wherein wireless communication is conducted between an AP and wireless terminals A and B using a single space.
First, the wireless terminal B transmits a Data1 signal addressed to the AP (TB111). The AP, upon receiving a Data1 signal normally, waits an SIFS interval, and subsequently returns an ACK signal (A111). The IEEE 802.11 standard at this time, after the ACK signal return, stipulates that the duration time included in the ACK signal should be set to 0 so as not to reserve transmission time for the next wireless signal, so that both the AP and the wireless terminals can equally transmit the wireless signal. By contrast, in the method described in patent document 1 (JP-A-2006-166114), this duration time is to be set to a value corresponding to, for example, the transmission time for the expected transmission of a subsequent Data2 signal and the transmission time for the ACK signal in response thereto. In other words, the AP reserves the transmission of the next wireless signal.
Upon receiving an ACK signal having a non-zero duration time, the wireless terminal A, in accordance with the IEEE 802.11 standard, configures the above-described non-zero duration time in its built-in clock (referred to hereinafter as NAV). Until the NAV value, which decreases with passing time, reaches zero, the wireless terminal A will not transmit data. However, the wireless terminal B to which the ACK is addressed, in accordance with the IEEE 802.11 standard, does not configure its NAV with the above-described duration time. Consequently, only the AP and the wireless terminal B become able to transmit, and after the waiting time, one of the two devices obtains transmission rights. Then, the general-purpose wireless terminal B waits a (DIFS+random number) interval (TB112), while the AP waits, for example, a DIFS interval. For this reason, the probability that the AP will obtain transmission rights is increased. It is supposed at this point that the AP obtains transmission rights and subsequently transmits a Data2 signal addressed to the wireless terminal-A (A112).
The wireless terminal A, upon receiving a Data2 signal normally, waits an SIFS interval, and subsequently returns an ACK signal and notifies the AP that a Data2 signal was received normally (TA111). When the transmission of this ACK is completed, the AP and the wireless terminals A and B, in accordance with a normal transmission rights acquisition method, respectively wait (DIFS+random number) intervals (A113, TA112, TB113.)
The case wherein the AP obtains the rights for the next transmission, and furthermore wherein the AP grants priority transmission rights to the wireless terminal B, will now be described. The AP transmits invalid data (hereinafter referred to as null data) addressed to the wireless terminal B (S118). This null data may include a duration time corresponding to the transmission time for the wireless terminal B to transmit the next expected ACK signal and a Data3 signal, or a duration time corresponding to the above transmission time plus the transmission time for the AP to return the corresponding Data3 signal with an ACK signal.
The terminals other than the wireless terminal B, such as the wireless terminal A, respectively configure their NAVs with a duration time like the above-described, and do not transmit data until the NAV value reaches zero. If the AP itself also configures its NAV with the above-described duration time, in the same way the AP do not transmit data until its NAV reaches zero.
On the other hand, the wireless terminal B, upon receiving Null data like the above-described, waits an SIFS interval, and subsequently returns an ACK signal notifying the AP that Null data was received normally (S118). Additionally, in the case where the wireless terminal B has transmission data, the wireless terminal B waits a (DIFS+random number) interval, and no wireless signals are received from other devices, transmits a Data3 signal (S119). In this case, since the other devices, such as the wireless terminal A and the AP, do not transmit data during the duration time, the possibility that the wireless terminal B will be able to transmit the next signal is high. In other words, as a result of the control process wherein the AP transmits null data including a non-zero duration time, the general purpose wireless terminal B gains priority transmission rights and interference is prevented.    Non-patent document 1: “Wireless LAN Medium Control (MAC) and Physical Layer (PHY) Specifications” ANSI/IEEE Std 802.11, 1999 Edition    Patent document 1: Japanese Unexamined Patent Application Publication No. 2006-166114